1. Field of the Invention:
The present invention relates to a compression ratio changing device using an eccentric bearing for an internal combustion engine in which the eccentric bearing is interposed between a piston-pin and a connecting rod and the compression ratio of the engine is changed according to the rotation of the eccentric bearing.
2. Description of the Prior Art:
In an Otto-cycle internal combustion engine it is desirable to increase the compression ratio, because an increased compression ratio brings about improved fuel efficiency as well as increased torque. However, excessively increasing the compression ratio increases probability of knocking in the engine when the gas in the combustion chamber is adiabatically compressed and the temperature of the gas rises. Knocking is liable to occur under high engine loads when an amount of air introduced into the combustion chamber is large and is less liable to occur under low engine loads when an amount of air introduced into the combustion chamber is small. Therefore, it is desirable to make the compression ratio changeable according to engine loads so that the compression ratio may be high under medium and low engine loads and may be low under high engine loads.
To satisfy these requirements various proposals have been made concerning compression ratio changing devices for an internal combustion engine. Japanese Utility Model Publication SHO No. 58-137832 discloses an eccentric bearing-type device in which the eccentric bearing having a cylindrical inside surface eccentric with respect to a cylindrical outside surface is installed between the piston-pin and the connecting rod. When the connecting rod rotates, the relative position of the piston with respect to the connecting rod is changed and the compression ratio of the engine is varied.
To explain the problems in the conventional eccentric bearing-type compression ratio changing device, the structure of the conventional device will be explained while referring to FIGS. 21 and 22. Eccentric bearing 6 having outside surface 6a and inside surface 6b is rotatably interposed between piston-pin 4 and the surface of small end hole 5 of connecting rod 3. Lock hole 9 is formed in eccentric bearing 6 and lock-pin hole 7 is formed in connecting rod 3. The centers of lock hole 9 and lock-pin hole 7 are included in a common plane which is at a right angle with respect to the axis of eccentric bearing 6. Lock-pin 8 is slidably inserted into lock-pin hole 7 to be able to move into lock hole 9. When lock-pin 8 in lock-pin hole 7 moves toward lock hole 9 and one portion of lock-pin 8 comes into engagement with lock hole 9, the rotation of eccentric bearing 6 is locked and when lock-pin 8 moves in the direction apart from lock hole 9 and disengages with lock hole 9, the rotation of eccentric bearing 6 becomes free.
When the rotation of eccentric bearing 6 is locked, the compression ratio is fixed. Since lock hole 9 is provided at such a position that lock hole 9 receives lock-pin 8 where the thickest wall portion of eccentric bearing 6 comes to the lowest position with respect to the axis of eccentric bearing 6, the fixed compression ratio is high. When the rotation of eccentric bearing 6 is unlocked, eccentric bearing 6 rotates around its axis receiving the moment which is produced by the loads on piston 2 and the arm of eccentricity of eccentric bearing 6. The loads on piston 2 include the combustion pressure, the compression force of the gas inside of cylinder block 1 and the inertia force of piston 2. When piston 2 is at T.D.C. of the compression stroke, eccentric bearing 6 rotates and naturally takes the rotational position in which the thickest wall portion of eccentric bearing 6 comes to the highest position with respect to the axis of eccentric bearing 6 and the compression ratio becomes low. In this way the compression ratio is changed by locking and unlocking the rotation of eccentric bearing 6 by means of lock-pin 8.
The driving of lock-pin 8 is performed by pressurized oil which is selectively supplied through locking oil path 10 and unlocking oil path 11 formed in connecting rod 3. When locking oil path 10 is pressurized, lock-pin 8 is driven toward lock hole 9, while unlocking oil path 11 is pressurized, lock-pin 8 is driven apart from lock hole 9.
To make the engaging of lock-pin 8 with lock hole 9 smooth, guide groove 13 is formed in the radially outer portion of eccentric bearing 6. Guide groove 13 extends circumferentially and the center of guide groove 13 is in the plane which includes the center of lock hole 9. Guide groove 13 begins at an outside surface of eccentric bearing 6 apart from lock hole 9 and becomes gradually deep toward lock hole 9 where guide groove 13 terminates. The portion of the surface of lock hole 9 opposing guide groove 13 constitutes colliding surface 14 which collides with lock-pin 8 and stops the rotation of eccentric bearing 6, thereby preventing lock-pin 8 from jumping lock hole 9 and allowing lock-pin 8 to enter lock hole 9 reliably.
However, in the compression ratio changing device having the above-mentioned structure there are the following problems.
When the compression changing mechanism is changed from the low compression ratio to the high compression ratio, lock-pin 8 is pushed toward lock hole 9 and engages with lock hole 9. In this instance, if the movement of lock-pin 8 toward lock hole 9 is not enough and accordingly lock-pin 8 collides with only the outer end portion of colliding surface 14 of eccentric bearing 6, the colliding load per a unit area of the load receiving portion of colliding surface 14 is very large and the outer end portion of colliding surface 14 will be deformed to protrude outward from outside surface of 6a eccentric bearing 6 and/or will be deformed to protrude into lock hole 9.
If the deformed portion of eccentric bearing 6 protrudes outward from the outside surface 6a of eccentric bearing 6, free rotation of eccentric bearing 6 will be deteriorated and it becomes difficult to obtain smooth changing of compression ratio and a desirable compression ratio in accordance with engine loads. If the deformed portion of eccentric bearing 6 protrudes into lock hole 9, a smooth entry of lock-pin 8 into lock hole 9 and smooth disengagement of lock-pin 8 from lock hole 9 will be deteriorated and a smooth changing of compression ratio will not be obtained.